1. Field of the Invention
The present invention relates to a process for producing a resin possessing low gas-permeability and excellent impact resistance and thermal stability, which comprises graft polymerizing, in two stages, a monomeric mixture of acrylonitrile and acrylic ester in the presence of a copolymer of 1,3-butadiene and acrylic (or methacrylic) ester.
2. Prior Art
The resin composition prepared according to the process of the present invention has the following advantageous characteristics.
(1) A shaped article formed from the resin composition has excellent impact resistance. Particularly, the resin composition of the present invention always makes it possible to produce a shaped article having high impact strength under any processing conditions.
(2) The resin composition can be easily formed into a shaped article which has good thermal stability and excellent processing property and is free of color. Particularly, due to no evolution of an offensive smell during the processing, the resin composition is suitable as a material for packaging foodstuffs. When a shaped article made from this composition is used for packaging foodstuffs, there is neither absorption of the smell of the packaging material into the foodstuffs nor change in the taste thereof.
(3) A shaped article made from the resin composition is glossy, colorless, and transparent and has a beatiful appearance.
(4) A shaped article made from the resin composition has very low permeability to gases such as carbon dioxide, oxygen and steam.
Heretofore, in order to improve the impact resistance of an acrylonitrile-based resin which is intended to be used as a gas barrier, there has been practiced a process which comprises graft-polymerizing a relatively large amount of monomers comprising mainly acrylonitrile in the presence of a relatively small amount of a rubbery trunk polymer.
However, the graft polymer obtained according to this process exhibits only a relatively low impact resistance and does not satisfactorily exhibit the intended properties. We have studied the possible causes for this. As a result, we have found that there is apparently present a certain relationship between the compatibility between the rubbery trunk polymer and the branch polymer selected and the properties of the resulting graft polymer and that the compatibility between the two polymers largely contributes to the properties of the graft polymer.
More specifically, as the compatibility between the rubbery trunk polymer and the branch polymer is poor, the degree of grafting of the branch polymer is low, and the resulting graft polymer has a non-uniform shape in the particles thereof, for which reason the graft polymer exhibits only a low impact resistance. That is, in the case where a monomeric mixture (for forming a branch polymer) consisting mainly of acrylonitrile is polymerized in the presence of particles of a trunk polymer consisting mainly of a conjugated diene, the shape and nature of the resulting graft polymer depend upon the degrees of compatibility between the trunk polymer and the branch polymer.
When the latex particles of a graft polymer obtained from the emulsion polymerization of the trunk polymer and the branch polymer of various degrees of compatibility thereof were examined under an electron microscope and the shapes of these particles were comparatively observed, it was found that in the case of a good compatibility the added monomers form a shell uniformly covering the surface of a core consisting of a spherical trunk polymer, that is, a graft polymer having a uniform coating is produced, while in the case of a poor compatibility the added monomers non-uniformly coat the surface of a spherical trunk polymer whereby a distorted shell is formed thereon, that is, a graft polymer having a non-uniform coating is produced.
It has also been found that a graft polymer comprising the trunk polymer with the uniform coating formed thereon has higher degree of grafting and excellent impact resistance and thermal stability, while a graft polymer comprising the trunk polymer with the non-uniform coating formed thereon is inferior to the former polymer with respect to these properties.
That is, the latter polymer has lower impact resistance and is not satisfactory with respect to practical strength. Further, it tends to be colored yellow or brown during the processing of its melt, and at the same time it gives off an offensive odor.
If a shaped article made from a graft polymer of such poor quality were to used to package foodstuffs, cosmetics, medical supplies and the like, the offensive odor of the article would infiltrate the content of the package, thereby reducing the value thereof to great extent, which would be unfavorable from the standpoint of practical use. However, with a graft polymer consisting mainly of acrylonitrile which is mainly intended for affording gas barrier property, the difference in compatibility between the trunk polymer and the branch polymer is, by itself, determined depending upon the composition of the two materials. Further, another matter which should be taken into consideration is that the composition of the two materials cannot be changed to a great extent in order to obtain a resin having the objective property. That is, in order that a graft polymer consisting of acrylonitrile possess the desired gas barrier property, the branch polymer constituting the major portion of the resin must have a composition such that the predominant portion of the component is acrylonitrile for the purpose of decreasing the gas permeability of the resin. Also, use is made of a rubber copolymer consisting mainly of a conjugated diene as a trunk polymer for the purpose of imparting impact resistance to the resin. In this case, a predominant amount of the conjugated diene is used so as to provide the desired impact resistance. A combination of a rubber polymer component and a grafting monomer component is thus selected so that the desired properties will be developed in the resultant graft polymer.
Accordingly, since compositions of the components are restricted within a certain range in order to attain desired properties, it is impossible to bring the compatibilities of both the components in complete agreement, and the difference cannot be disregarded in order to attain desirable properties such as impact resistance.